The present invention relates to a method for producing a toner for electrostatic image development and, more specifically, it relates to an economically advantageous method for producing a toner for electrostatic image development, causing less fogging and capable of providing a satisfactory image quality.
A toner for electrostatic image development (hereinafter simply referred to as "toner") comprises resin particles having particle size of 1 to 50 .mu.m, preferably, an average classified diameter of 3 to 15 .mu.m in which a colorant and, if required, toner property-imparting agents (for example, a charge controlling agent and magnetic particles) are dispersed in a thermoplastic resin as a binder resin. The toner is used as a one-component developer containing the toner alone or as a two-component developer containing a mixture of the toner with a carrier.
Generally, the toner is produced by mixing starting toner materials, kneading them in a melt extruder or the like and then cooling and grinding them. In production of the toner, the grinding process is a particularly important step for giving an influence on the characteristics of the toner. Namely, an excessively ground toner causes fogging, whereas an insufficiently ground toner deteriorates image quality.
The grinding process for producing the toner usually comprises three steps, namely, coarse crushing step, medium crushing step and fine pulverizing step. Such grinding process is proposed, for example, in Japanese Patent Application Laid-Open (Kokai) No. 58-42057 (1983). In the grinding process as described in the publication, a toner material extruded from a melt extruder into a plate shape and then cooled to solidify is at first crushed by a hammer crusher, then crushed to a medium size by an impact crusher and then further pulverized finely by a jet pulverizer. Subsequently, a classifying treatment is applied to recover a toner.
However, since the grinding process described above consumes much energy in the medium crushing step and the fine pulverizing step, it can not be considered as an economically advantageous method. Further, in the pulverization using the jet pulverizer, an over-pulverized toner is formed by as much as 15 to 40% by weight and, accordingly, the over-pulverized toner tends to intrude into final toner products and the productivity becomes poor because the over-pulverized toner has to be removed and, in addition, an additional energy is required for re-using the over-pulverized toner once removed.
In view of the above, the present inventors have found that by using a specific pulverizer, a toner for electrostatic charge development, causing less fogging and capable of providing satisfactory image quality can be produced with less occurrence of over-pulverized toner, at a satisfactory productivity and with an economical advantage. The present invention has been accomplished based on the finding.